Replace subprocess with python-libjuju calls

[zxhdaze]

There are few places in the code, which rely on running subprocess and using juju.

It needs to be replaced with appropriate python-libjuju calls.

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Imported from Launchpad using lp2gh.

• date created: 2021-08-27T19:38:34Z

• owner: smigiel-dariusz

• assignee: None

• the launchpad url

[zxhdaze]

(by zzehring)
Currently, python-libjuju does not implement a couple of key functions we need for the tool: retrieving all controllers and creating controller backups.

Here are the GH Issues for implementing those features:

controllers: juju/python-libjuju#529
controller backup: juju/python-libjuju#529

Once those are implemented, we can remove the subprocess calls and add the lib calls.

[zxhdaze]

(by jfguedez)
The upstream issues have been resolved. The current code in master uses the API for creating controller backups.